Green nanocoating-based polysaccharides decorated with ZnONPs doped Egyptian kaolinite for antimicrobial coating paper

Paper coating plays an important role in the paper properties, printability and application. The nanocoating is a multifunction layer that provides the paper with unique features. In this work, nanocoating formulas were prepared using a green method and component. The nanocoating formulas were based on biopolymers nanostarch NSt and nanochitosan NCh (NCS) decorated with Egyptian kaolinite Ka doped with zinc nanoparticles NCS@xka/ZnONPs (x represents different ratios) support for multifunctional uses. The nanocoating formulas were characterized using a physiochemical analysis as well as a topographical study. FTIR, XRD, SEM and TEM techniques were used. Additionally, the antimicrobial activity of the tested samples was assessed against six microorganisms including Gram-negative and Gram-positive bacteria. The prepared nanocoating formulas affirmed excellent antimicrobial activity as a broad-spectrum antimicrobial active agent with excellent activity against all representative microbial communities. The nanocoating with the highest ratio of Ka/ZnONPs (NCS@40 ka/ZnONPs) showed excellent antimicrobial activity with an inhibition percentage of more than 70% versus all microorganisms presented. The paper was coated with the prepared suspensions and characterized concerning optical, mechanical and physical properties. When Ka/ZnONPs were loaded into NCS in a variety of ratios, the characteristics of coated paper were enhanced compared to blank paper. The sample NCS@40 ka/ZnONPs increased tensile strength by 11%, reduced light scattering by 12%, and improved brightness and whiteness by 1%. Paper coated with NCh suspension had 35.32% less roughness and 188.6% less porosity. When coated with the sample NCS@10 ka/ZnONPs, the coated paper's porosity was reduced by 94% and its roughness was reduced by 10.85%. The greatest reduction in water absorptivity was attained by coating with the same sample, with a reduction percentage of 132%.

www.nature.com/scientificreports/ Characterizations. Green nanocoating characterizations. The prepared samples were characterized, including topographical and physiochemical characterizations. The topographical study involved a High-resolution transmission electron microscope (HRTEM, JEOL 2010, Japan) operating at 300 kV. It was used to examine the particle shape and size of the prepared nanoparticles and select areas for electron diffraction (SAED). Scanning electron microscopy (SEM), Quanta FEG 250, FEI, Republic of Czech) linked with energy dispersive X-ray analysis (EDX; Model Quanta 250 FEG, Field Emission Gun) (of note this SEM was used for coating paper sheet). Additionally, the physiochemical characterizations were studied via Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy "Spectrum Two IR Spectrometer-PerkinElmer, Inc., Shelton, USA". Spectral analysis was obtained at 32 scans and 4 cm −1 resolutions in wavenumbers ranging from 4000 to 400 cm −1 . The x-ray diffraction (XRD) patterns of the different samples were investigated using a Diano X-ray diffractometer (Philips) provided with a Cu Kα radiation source (λ = 0.15418 nm), energized at 45 kV, as well as a generator (PW 1930) and a goniometer (PW 1820).
Coated paper samples characterization. The properties of coated paper samples, including the prepared suspensions, were evaluated using standard tests. Table 1 represents the coated paper properties, instruments, and the standard methods followed for coated paper characterization.
Antimicrobial study. Microbial strains and growth conditions. The antimicrobial activity of tested samples was assessed against six microorganisms, including Gram-negative bacteria (Escherichia coli ATCC25922 & Pseudomonas aeruginosa ATCC 27853), Gram-positive bacteria (Staphylococcus aureus ATCC25923 & Bacillus subtilis ATCC6051), unicellular fungi (Candida albicans ATCC90028, and filamentous fungi (Aspergillus niger RCMB 02724). Bacterial strains were cultivated on nutrient broth at 37 °C for 24 h, while fungal strains were grown on malt extract broth medium then incubated for 3-5 days at 28 °C ± 2; and then kept at 4 °C for further use [37][38][39][40] . As mentioned in our previous work 41 , the cell formation unit counting (CFU) approach was applied. About 0.1 g of the sample weight was used. The antibiotics Streptomycin and Griseofulvin were used as standard a broad spectrum antibacterial and antifungal, respectively.

Results and discussion
Nanocoating characterizations. Characterizations of nanocoating formulas were included the physicochemical analysis to investigate the interaction between nanocoating complements as well as the behavior of Ka before and after being added into Ka/ZnONPs and other formulations. Moreover, the topographical study affirmed the surface texture and performance of the nanocoating that assist in the prediction of nanocoating role in improving the paper surface and other properties.
Topographical study of green nanocoating. The prepared green nanocoating was characterized topographically using SEM and EDX chart against the not modified kaolinite as shown in Fig. 1. The SEM image of kaolinite ( Fig. 1A) was performed as a random particle with a uniform shape and the brightness is close to inorganic shine. Otherwise, the NCS@20 ka/ZnONPs were observed in Fig. 1B with more details in comparison with the blank kaolinite with clear rods for ZnONPs. In addition, the EDX chart of kaolinite ( Fig. 1C) illustrated the atomic content as mainly Al and Si. This observation is fitted with kaolinite chemical composition. In this context, the EDX chart of NCS@20 ka/ZnONPs was assigned the same composition with the presence of Zn ion. These observations indicate that the doping process was carried out. Nonetheless, Fig. 2 illustrated the TEM images and SEAD diffraction of Ka/ZnONPs and NCS@20 ka/ ZnONPs. The Ka/ZnONPs TEM image ( Fig. 2A) was shown as a plate structure aggregated with ZnONPs rods. Moreover, the SAED pattern of The Ka/ZnONPs (Fig. 2C) affirmed a high crystallinity behavior with rings arranged with sharp and shining spots distributed regularly on the rings. Likewise, the NCS@20 ka/ZnONPs www.nature.com/scientificreports/ TEM image was illustrated in Fig. 2B with a nice performance that related to the stabilization of the polysaccharide which fills the plats gabs of Ka/ZnONPs and appears as massive particles aggregated together. Furthermore, the SAED pattern of NCS@20 ka/ZnONPs (Fig. 2D) confirmed the image appearance, and the crystallinity performance was changed with a similar polycrystalline diffraction pattern. These observations affirmed that the polysaccharides affected the structure of Ka/ZnONPs that penetrate the plates form and stabilized the performance of the formulated structure.
Physiochemical characterizations. The physiochemical characterizations of the nanocoating neat component and different formulas of nanocoating were included in FTIR and XRD. The FTIR spectra were illustrated in Fig. 3. The nanocoating biopolymer components including NCh and NSt, as well as the NCS formula, were observed in Fig. 3A. NCh spectrum was observed, a characteristic band at 3259 cm −1 that corresponded to hydroxyl groups stretching. Additionally, the band of C-H stretching vibration was assigned as two small bands at 2934 and 2879 cm −1 due to the nanoformulation of chitosan 35 43,44 . On the other hand, the formulation of the Ch/St templet of nanocoating affects the FTIR spectrum, especially in both groups, namely, OH and CH. In particular, the OH band was observed to be sharper, as well as the frequency of the CH band, which was reduced to a lower position. This occurrence could be related to interactions between biopolymers. Furthermore, the Ka spectrum exhibited characteristic bands at 3689 and 911 cm −1 that were attributed to hydroxyl stretching bands to the inner surface, with hydroxyl groups oriented towards the vacant sites in the external layers of the kaolinite structure 34 . Otherwise, zinc oxide doping was observed in new bands at 1555, 685, and 532 cm −1 that were due to the incorporation of ZnO in the plating structure of Ka 45 . However, the incorporation of Ka/ZnONPs into nanocoating formulas with different concentrations that were assigned as new bands at 3693, 910, and 545 cm −1 was attributed to Ka, ZnO-Ka new bond and ZnONPs FTIR fingerprint region bands, respectively. Indeed, the intensity of these bands was going in a parallel direction with Ka/ZnONPs concentration. The crystallography study was shown in Fig. 4. The NCh crystallographic pattern was observed as a typical crystallography of NCh as reported in our previous work 14,46,47 with high crystallinity and a sharp characteristic peak at around 20°4 8 . In addition, the NSt pattern showed amorphous behavior with characteristic peaks at 16.9,   51,52 . Additionally, of ZnONPs effect on the Ka pattern, showed the disappearance of a peak at 20° as well as the appearance of small peaks at 38 and 51° that were due to the doping of ZnONPs into the Ka structure 53,54 . In addition, the different formulas of nanocoating patterns confirmed that the polysaccharide peak was around 10°. However, the crystallographic pattern of nanocoating formulas was similar due to the dominance of inorganic materials in the XRD pattern over polymers.
Paper coating topography. Figure 5 (upper) showed the high resolution photos of coated paper sheets were comparison with the blank one. The black paper sheet was observed as a traditional paper sheets with some blueness color that reduced to yellowish white after coating with the different formulas. Moreover, the yellowing color was reduced to clear whit in sample NCS@40 ka/ZnONPs due clay and ZnoNPs white color. On the other side, the Fig. 5 (lower) was illustrated SEM images of the paper coated with different formulas of nanocoating compared with plank paper as well as mapping of the Zn ion of the highest formula Zn content. At first glance, the fibers of paper were observed, as seen in all SEM images. Blank paper fibers appeared to have a typical performance for paper. Additionally, the paper coating with the NCS@10 ka/ZnONPs SEM image illustrated a significant appearance of not only the filling of gaps between fibers but also the fiber surface behaviors. In this context, the SEM images for all applied nanocoatings with different ratios of KA/ZnONPs were assigned, whereas the metallic particles were seen in the fiber gaps with different ratios according to nanocoating Ka/ZnONPs. Herein, the fibers of all coated papers were observed coated with transparent and thin coatings. In addition to, the mapping image of Zn ion distribution confirmed that the Zn ion was disturbed homogenously.
Antimicrobial study. The nanocoating formulas were tested against six microbial strains, which are representative of the most infectious microorganisms, as shown in Table 2 and comparison with a standrads antimicrobial agents namely; streptomycin as antibacterial and griseofulvin as antifungal. The efficacy of NCS@10 ka/ ZnONPs, NCS@20 ka/ZnONPs, and NCS@40 ka/ZnONPs showed an obvious relation between the Ka/ZnONPs   www.nature.com/scientificreports/ percentage contained in the nanocoatings. Moreover, the nanocoating with the highest ratio of Ka/ZnONPs (NCS@40 ka/ZnONPs) showed excellent antimicrobial activity against all presented microorganisms. These results related to broad spectrum antimicrobial activity. Herein, the antimicrobial activity of nanocoating was gained from NCh, Ka, and ZnONPs. Furthermore, the synergetic effect between the three previously mentioned biological active components played a limited role in improving the efficiency of the antimicrobial activity of the prepared nanocoating. In sum, the sample coated with NCS@40 ka/ZnONPs (containing the highest percentage of ZnONPs) was presented an excellent antimicrobial activity against Gram positive and negative bacteria in compared with the standard antibiotic. Otherwise, the effect of NCS@40 ka/ZnONPs against the unicellular fungi in comparison with the standard drug was recorded a close value. These observations were due to the excellent antibacterial activity and moderate antifungal activity of ZnONPs 55-57 .  for high-brightness paper has compelled paper manufacturers to devise novel techniques for enhancing the brightness and whiteness of coated paper. Pigment coating is commonly used to improve the optical properties of paper and paperboard, such as brightness, whiteness, and gloss. These optical properties are crucial for the end user and also determine the final price of coated paper 4 . The optical properties of paper products are critical parameters, primarily due to their aesthetic qualities, but they also play a crucial role in print or writing showing through paper products. The properties are defined by reflectance, absorption, and light transmission through paper 34 . Figure 7 represents the optical properties of paper coated with different nanocoatings.
Brightness. Coating the blank sample with NCh or NCh/NSt did not affect the paper's brightness, as shown in Fig. 6A. When Ka/ZnONPs were included in the coating suspensions, the brightness of the coated paper increased slightly. It increased as the percentage of Ka/ZnONPs increased compared to the blank sample. The percentage change reached 1% at the sample NCS@40 ka/ZnONPs.
Whiteness. Figure 6B shows that neither NCh nor NCh/NSt suspensions affected the whiteness of coated paper compared to a blank. The addition of Ka/ZnONPs to the coating suspension slightly enhanced the paper's whiteness. The percentage of increase reached about 1% only when Ka/ZnONPs were loaded with different ratios into NCS.
Opacity. Opacity is necessary to prevent printed text from appearing on the back of a sheet of paper. It is directly related to light scattering and the porous coating layer structure 34 . The results in Fig. 6C indicate that coating suspensions containing NCh reduced opacity by approximately 0.45%. In comparison to the opacity of the blank sample, the addition of NCS or NCS@10 ka/ZnONPs resulted in a very slight improvement. The addition of NSt, which is a rheology modifier, increased the viscosity of the solution 5 . Therefore, the opacity of samples containing NCS was increased.
Light scattering. NCh-containing coating suspension reduced light scattering by approximately 12%. The addition of NCS to the coating suspension resulted in a 1% decrease in light scattering compared to a blank sample. Figure 6D shows that when Ka/ZnONPs were loaded with different ratios into NCS, light scattering was reduced, and the reduction was proportional to their percentage. The percentage decreased to 6.6, 10, and 12% at the samples NCS@10 ka/ZnONPs, NCS@20 ka/ZnONPs and NCS@40 ka/ZnONPs, respectively, relative to the light scattering of the blank sample. The particle morphology influences light scattering via the number and size of air microvoids in the sheet. Due to the particle morphology difference between ZnONPs and kaolinite, as the percentage of ka/ZnONPs increased, the air voids in the packing structure of the coating layer decreased, resulting in less light scattering. The size press treatment was implemented to coat paper with ZnONPs using oxidized starch as a binder. ZnONPs were also used in combination with calcined clay to enhance opacity and reduce print through 58 .
Mechanical properties. The mechanical properties of coated paper are important in the examination of the bending and compressive deformation that coated paper undergoes in a printing press, e.g., paper handling and tunability. Figure 7 represents the mechanical properties of paper coated with various coating suspensions.
Tensile strength. By introducing NCh to the coating suspension, the tensile strength increased to 9.5%. But by adding NCS, the tensile strength decreased by 1% compared to the blank sample. This decrease continued when the coating suspension contained NCS@10 ka/ZnONPs (Fig. 7A), it reaching 7%. The tensile strength improved as the percentage of ka/ZnONPs increased at the samples NCS@20 ka/ZnONPs and NCS@40 ka/ZnONPs, it reached 6 and 11%, respectively.
Tensile energy absorption. Tensile energy absorption (TEA) of all prepared coated paper was increased compared to the blank sample (Fig. 7B). TEA increased by 30.3% by using NCh as a coating suspension. By forming a suspension containing NCS the percentage decreased but was still higher than that of the blank sample by Table 2. Antimicrobial activity of nanocoating formulas against microbial strains. *Antimicrobial activity % **This antibiotic is not applicable for this strain. Stretch. The stretch of all prepared coated paper was increased compared to the blank sample, as shown in Fig. 7C. The stretch of samples coated with NCh and NCS increased to 17.63% and 21.65%, respectively. The sample coated with NCS@10 ka/ZnONPs showed the maximum stretch, the increase reached 24.30%. The samples coated with NCS@20 ka/ZnONPs and NCS@40 ka/ZnONPs increased coated paper stretch by 21.7% and 22%, respectively, compared to the blank sample. Chitosan has many applications in the food industry, including antimicrobial film production and coatings. But it has limited mechanical and antimicrobial properties 23 .
Enhancing the antibacterial activity of Ch is achieved by the formation of CS/inorganic composites; thus, the combination of chitosan and ZnONPs develops new biomaterials with excellent antimicrobial activities. This kind of organic-inorganic hybrids not only improve antimicrobial activities, but lower the usage of ZnO and enhance biocompatibility 59 . Nanoclay inclusion in composite films, based on chitosan and nanoclays, led to enhanced mechanical properties. The mechanism of this improvement is related to the formation of intercalated and/or exfoliated composite structures [60][61][62] .
Nanocomposites exhibit increased barrier properties and mechanical strength compared to their native polymers and conventional composites. Nanocomposites at the nanoscale level result in a large interfacial area, or boundary area, between the biopolymers and nanoparticles. The large interface enabled the modification of www.nature.com/scientificreports/ molecular mobility and relaxation behavior, as well as the mechanical, thermal, and barrier properties of bionanocomposites. The increase in mechanical properties of bio-nanocomposite materials is due to the high rigidity of nanofillers as well as the excellent affinity between biopolymer and nanofiller at the interface 63 .
Physical properties. Roughness. Roughness describes the topography of the paper surface. It should be low to attain good printing properties. Coated paper roughness is affected by many factors, one of which is the morphology of the pigment particles. Figure 8A represents the effect of different coating suspensions on coated paper roughness. All coating suspensions caused a decrease in coated paper roughness compared to the blank sample. The roughness of paper coated with NCh suspension decreased by 35.32%. In the case of NCS, it decreased by 20.0%. ka/ZnONPs in coating suspension had less effect on coated paper roughness. NCS@10 ka/ZnONPs decreased the roughness by 10.85%, while NCS@20 ka/ZnONPs and NCS@40 ka/ZnONPs decreased the roughness by about 6%.
Porosity. Figure 8B represents the results of porosity measurements of paper coated with various coating suspensions. As with roughness, all coating suspensions caused a decrease in coated paper porosity compared to the blank sample. The greatest effect was shown by NCh suspension, which decreased the porosity by 188.6%. NCS decreased it by 134%. The porosity of paper coated with the samples NCS@10 ka/ZnONPs, NCS@20 ka/ ZnONPs and NCS@40 ka/ZnONPs decreased by percentages equal to 94.42%, 88.8%, and 82.85%, respectively.  www.nature.com/scientificreports/ During the coating application, the water leaks into the base sheet. As water is removed from a coating layer, the solid content increases, and a filter-cake layer begins to form on the paper web. When the coating film is set down uniformly to the paper web, the porosity and paper roughness decrease, in other words, the smoothness of the paper increases. The inherent platy structure of kaolinite pigment produces a dense and compact coat layer structure 34 . This high packing characteristic of clay pigment decreased upon increasing the percentage of ZnONPs. Consequently, paper roughness and porosity increased by increasing the percentage of ZnONPs in the combination of ka/ZnONPs. Water absorptivity. Figure 8C represents the water absorptivity of paper coated with various coating suspensions. It was measured by the Cobb test. Water absorbency is a complex and dynamic phenomenon that is influenced by many physical, chemical, and morphological aspects of cellulosic fibers (e.g., surface composition, surface roughness, bulk composition, charged groups, and fiber web porosity 64 . The Cobb value indicates the ability of paper to absorb water. Small Cobb values mean high water resistance in the paper. There is a decrease in Cobb values of all the coated paper compared to the blank paper sample. The minimum decrease in water absorptivity was obtained by coating the paper with NCh suspension. It reached 73%. The maximum decrease was achieved by coating with the sample NCS@10 ka/ZnONPs, the reduction percentage reached 132%. The decrease attained by coating the paper with NCS was 107%. By increasing the percentage of ka/ZnONPs, the www.nature.com/scientificreports/ Cobb value increased but was still lower than that of the blank paper sample. The samples coated with the suspensions containing NCS@20 ka/ZnONPs and NCS@40 ka/ZnONPs achieved decreases in Cobb values of 104% and 101.7%, respectively.

Conclusion
Egyptian kaolinite doped ZnONP was loaded onto nanochitosan and nanostarch to formulate a green nanocoating for multifunction paper. This nanocoating was supported by excellent physical, mechanical, optical, and antimicrobial capabilities. Then, these formulas performed antimicrobial activity against six microorganisms that are representative of most poison microorganism families, and the sample containing the highest percentage of ka/ZnONPs represented excellent antimicrobial activity with an inhibition percentage of more than 70% against all tested microorganisms. The properties of coated paper were improved in comparison to blank paper when Ka/ ZnONPs were put into NCS in various ratios. The sample NCS@40 ka/ZnONPs improved brightness and whiteness by 1%, decreased light scattering by 12%, and increased tensile strength by 11%. NCh suspension-coated paper showed 188.6% less porosity and 35.32% reduced roughness. Loading of Ka/ZnONPs in NCS reduced porosity by 94% and roughness by 10%. Coating with the sample NCS@10 ka/ZnONPs led to the greatest reduction in water absorptivity, with a reduction percentage of 132%. Finally, the nanocoating formulas successfully combine the development of antimicrobial properties with the enhancement of the qualities of paper, which are crucial for using paper for printing and packaging.

Data availability
The data and materials were mentioned in the manuscript and the data was available upon request from the corresponding author.